Optimum Highway Speed

Discussion in 'Gen 4 Prius Fuel Economy' started by Zardoz, Sep 22, 2017.

  1. bbald123

    bbald123 Thermodynamics Law Enforcement

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    Hard to believe, isn't it?
     
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  2. Tideland Prius

    Tideland Prius Moderator of the North
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    Wayne in the US so it's US gallons.

    I got 52mpg on a trip that included interstate driving (60 and 70mph limits). Another 60 mile trip with speed limits between 40mph and 50mph netted me 64 mpg.

    Note that as mpg increases in value, each additional unit is a smaller piece of the pie so 10 mpg from 60 to 70mpg is actually just 0.56L/100km. Or 0.24 gal/100 miles if I did the conversion correctly. 20 to 30mpg is a difference of 3.92L/100km or 1.66 gal/100 miles; a bigger savings.

    So even if you got 60 mpg instead of 78 (which I think is a large enough difference to count as "nowhere near"), you're actually not as far off as you think.
     
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  3. fuzzy1

    fuzzy1 Senior Member

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    Note that he got even better on a 2017 Prime, 83 US MPG at 50 mph:
    Steady State Speed vs Fuel Economy results | Page 13 | CleanMPG

    But do remember than nearly all cars get better MPG in Wayne's hands than in the hands of us ordinary mortals.
     
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  4. KV55

    KV55 Member

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    Wayne's chart reflects what I have been seeing, around 52mph steady state (where it goes to a state of ICE energy to wheels and doesn't tend to switch back and forth with the electric motor) I get what looks like 90mpg UK. The Gen 2 mantra was 70mpg (UK) at 50mph, 60 at 60 and 50 at 70.
     
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  5. first_superior_prius

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    I consistently get readings very similar to the chart. Flat ground, no headwind, warmed up engine, steady speed at 50 is very possible to achieve 75-80 MPGs

    Most people don't achieve that though, because they aren't staying at a steady speed, and their driving behavior is not conducive to getting good mileage. Accelerating too fast (including braking instead of coasting) is what kills your mileage the most
     
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  6. bbald123

    bbald123 Thermodynamics Law Enforcement

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    If the chart reflects using hypermiling techniques, I withdraw my criticism.
     
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  7. fuzzy1

    fuzzy1 Senior Member

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    I don't believe Wayne is any longer capable of not hypermilng.
     
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  8. bestmapman

    bestmapman 04, 07 ,08, 09, 10 and 16 Pri

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    As I read this, it refers to a steady state snapshot. It does not consider acceleration and any other variable. So with that interpretation I think it is very close. To me it is almost unobtainable due to start up, acceleration, hills, slower vehicles, stop lights, etc. All those will take away from actual numbers.

    LOL
     
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  9. Mendel Leisk

    Mendel Leisk Senior Member

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    To belabour my point: regardless of the mpg numbers, the chart is uniformly steep.
     
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  10. first_superior_prius

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    But, in the context of OP, who asked about highway speed, it should be pretty spot on. Assuming, like any reasonable person, they are using cruise control on the highway, and their highway does not have stoplights
     
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  11. first_superior_prius

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    It's not reflecting hypermiling to much of any degree. It's reflecting what the vehicle gets when it is at level speed on flat ground

    I think you're conflating real-world driving on all roads, highway speed, and hypermiling

    Since the OP asked about MPG and speeds on highway, I think we can assume the chart pretty accurately provides good contextual data for them to draw a conclusion
     
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  12. pilotgrrl

    pilotgrrl Senior Member

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    Or Chart Guy is driving downhill all the way.

    Posted via the PriusChat mobile app.
     
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  13. RCO

    RCO Senior Member

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    Started out from top of the world I imagine. :sneaky:
     
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  14. bbald123

    bbald123 Thermodynamics Law Enforcement

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    Then I restate my criticism.

    I watch indicated MPG quite closely and I don't consistently see 78 mpgi on flat ground with no car in front of me to give me an aerodynamic tow. With slight downgrade? Yes. With aerodynamic tow? Yes.

    I'd need to see a clear statement of data acquisition methodology and mpgi reporting rules. If these are peak numbers? Perhaps I could buy in.
     
  15. Mendel Leisk

    Mendel Leisk Senior Member

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    Yeah but choose your battles. If you knock 5~10 mpg off each data point, the graph has the same slope, just shifted down a bit. Which in turn illustrates: there is no sweet spot speed.
     
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  16. first_superior_prius

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    Here's a real-world snapshot from yesterday. I'll give you my methodology:

    I took a secondary highway home from work. Almost completely flat. 50 MPH speed limit, just under a 20 mile drive, 3 stoplights.

    From a stop, I reset Trip A, accelerated to 50 MPH, and set cruise control. A/C was on at 70 degrees with 2 bars on the fan.There was zero traffic so I remained at that speed. It was 8 miles to the next stoplight. It was red, so I coasted about 1/10th of a mile to it, stopped and took this picture. Average speed 49 MPH, due to the slight bit of acceleration at the start and the coast to stop, plus the time at 0 MPH it took to take the picture. (Didn't want to be unsafe and take a pic at speed)

    [​IMG]


    My display reads 5% high; corrected to real MPG that is a shade over 81 MPG.

    I repeated the acceleration to 50 MPH, then setting cruise, thru the next 2 lights (both red, so I waited at them a bit). Pulled off the highway into a gas station after the last light to take this pic. You can see that the average speed is in the mid-40s due to the wait times at the lights, but I was on cruise at 50 MPH the whole rest of the way, flat ground, no traffic.

    [​IMG]

    Corrected for display inflation, that's 79 MPG


    Note that you have to drive for awhile at steady state to get a true average. Especially at 50 MPGs, the EV mode will kick on for a mile or more at a time, greatly increasing MPGs. Then the battery will get recharged, greatly lowering MPGs. Depending on where you look at the cycle, you will get a very skewed idea of long-term average of steady state speed. You can see the cycle it in the first picture if you look at the 1-mile bars from 0-10 miles.
     
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  17. bestmapman

    bestmapman 04, 07 ,08, 09, 10 and 16 Pri

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    Thanks for the experiment. Interesting results. Seems to confirm the chart.
     
  18. MelonPrius

    MelonPrius Active Member

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    I second this. Thank you, First_superior_prius. I appreciate it when board members test out different mpg theories and share their results. This includes the valuable mpg vs mph charts. The vehicle's fuel economy tracking system makes it easy.

    I like to test out different routes and compare the mpg. If nothing else, it breaks up the monotony of every day driving. Based on my experience, in order for a test to be completely accurate, you should do a round trip- at least in my town.
     
  19. bbald123

    bbald123 Thermodynamics Law Enforcement

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    Anyone can hit instantaneous and very short distance MPG numbers. Since the chart is claiming to plot mpg versus speed, instantaneous or very short distance numbers aren't useful.

    I suspect the chart posted mostly because it is a straight line. Take a look at the graph (from here) to see why I have my doubts. If increased in power requirements is a curve, the mpg drop should also probably be a curve.

    [​IMG]
     
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  20. first_superior_prius

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    I'm not sure I would call 8 miles a "short distance" or instantaneous...

    Note that the speeds in question from the CleanMPG graph are from 50 MPH to 70 MPH. If you look at that speed range from the chart you posted for a Jaguar, it is pretty darn close to linear. So I think your graph and Wayne's agree pretty well for that particular speed range
    [​IMG]
     
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